1. Field of the Invention
The present invention relates to a visual axis detection apparatus, and more particularly to a visual axis detection apparatus which irradiates an eyeball of an observer (photographer) with a light beam from light projection means, forms a first Purkinje image (cornea reflected image) due to reflected light from the eyeball and a pupil on an image sensor plane and detects the visual axis of the eyeball in accordance with positional coordinates of the images on the image sensor while properly controlling the accumulation time in detecting the light beam with the image sensor in order to permit high precision detection of the visual axis, and which is suitable for use in a still camera or a video camera.
2. Related Background Art
FIG. 9 shows a block diagram of a prior art visual axis detection apparatus.
Numeral 91 denotes a microprocessing unit (MPU) which carriers out various operations such as a visual axis calculation in accordance with positional information of a first Purkinje image and a pupil. Numeral 92 denotes a memory, numeral 93 denotes an interface circuit which has an A/D conversion function, numeral 97 denotes light projection means which projects an infrared ray, which is invisible to an observer, emitted from an infrared ray light emitting diode 97ato an eyeball (not shown) of the observer through a projection lens 97b, numeral 95 denotes a light emission control circuit which controls the light emission of the infrared light emitting diode 97a, and numeral 96 denotes a position sensor which detects the vertical/horizontal position of a camera when the visual axis detection apparatus is applied to the camera.
Numeral 94 denotes detection means which includes an image sensor 94a, a driver 94b and a lens 94c and focuses the first Purkinje image due to the reflected light from the eyeball and the pupil image onto the image sensor 94a through the lens 94c.
A method for detecting the visual axis-of the eyeball in FIG. 9 has been proposed by Japanese Laid-Open Patent Application No. 2-209125 or 2-264632, in which the visual axis is detected by using two positional information of the first Purkinje image (cornea reflected image) due to the reflected light from the eyeball of the infrared ray emitted from the light projection means 97, and the pupil center calculated from the contours of the pupil.
The infrared ray from the projection means irradiates the eyeball of the observer from the front, and the position of a virtual image of the infrared light emitting diode generated by the reflection by the front plane of the cornea, that is, a so-called first Purkinje image, is detected by the image sensor. The position at which the first Purkinje image is generated corresponds to the position of the pupil center when a rotation angle of the eyeball is zero (the optical axis of the eyeball) and it deviates from the pupil center as the eyeball rotates.
The deviation (distance) between the first Purkinje image and the pupil center is substantially proportional to the sine of the rotation angle of the eyeball. Accordingly, the distance is determined from the positional information of the first Purkinje image and the pupil center, and the rotation angle and the visual axis correction are calculated to determine the visual axis of the photographer.
The accumulation time of the image sensor when the image sensor senses the light beam is set to an appropriate time while taking into consideration various conditions such as the emission luminance of the infrared light emitting diode, the sensitivity of the image sensor, an S/N ratio and an external light which is normally ancitipated in photographing, and the image sensor accumulates the light beam for the predetermined accumulation time.
In the prior art visual axis detection apparatus, the accumulation time of the image sensor when the image sensor senses the light beam is set in accordance with the conditions described above and the light beam is sensed for the preset accumulation time.
As a result, various problems may arise depending on the illumination of an anterior portion of the eyeball. For example, if the illumination is low, the contrast (a difference between output signals) of the pupil and the iris is low and it is difficult to detect the contour of the pupil. On the other hand, when the illumination is very high, the image sensor saturates and the difference between the output signals for the first Purkinje image and the iris, which inherently has a difference, disappears and it is impossible or difficult to detect the first Purkinje image.